Storage and transportation of aluminium strip

ABSTRACT

The invention is particularly directed to the problem of creep deformation in a coiled aluminum strip, which occurs during coiling and for a period thereafter. The problem arises because the profile of the strip across its width is not flat, and is in fact usually thicker in the middle than at the edges (positive crown). To compensate for this, the invention provides that the spool onto which the coil is wound is adapted to provide more support to the strip in the center than at the edges, by making the central portion of the spool of a greater diameter than the end portions of the spool. A strip having a positive crown which is wound onto such a spool was found to exhibit significantly reduced creep strain, leading to reduced creep deformation.

[0001] The present invention relates to a spool suitable for use in thestorage and transportation of strip material made of aluminium or analloy thereof, and to a method of coiling such material on a spool.

[0002] Aluminium strip material, such as that used in lithographicprinting, is coiled under tension on large steel or fibre spools forstorage and transportation. The spool is a large cylinder that has auniform outer diameter and a length sufficient to completely support thewidth of the strip material, often in practice extending beyond thestrip for a short distance either side. It is known that the coiling ofaluminium strip material can affect the flatness of the strip. Aluminiumstrip material that was flat immediately before it was coiled onto aspool can become off-flat as the strip creeps under the uneven stressesthat arise across the width of the strip. Aluminium presents aparticular problem in coiling because it is much more prone to creepthan, for example, steel.

[0003] The non-uniform stresses across the width of the strip when it iscoiled arise from the fact that the thickness of the strip variesslightly across the width of the strip, with the strip usually beingslightly thicker in the middle than at the edges (a positive crown).This variation in thickness results in the coil being slightlybarrel-shaped, i.e. the coil has a larger diameter at its middle than atits edges. This further results in the middle of the coil carrying moreof the coiling tension than the edges.

[0004] The manufacturing process for aluminium strip generally tries toensure that the strip does have a positive crown since strip with anegative crown (implying that the outer edges are thicker than thecentre) can result in unpredictable handling, particularly during laterfabrication processes. Because the manufacturing process is a multi-stepprocess, a margin of error needs to be built in to ensure that no partof the output has a negative crown. Thus the manufacturing process isset to deliberately provide a crown, typically such that the thicknessin the central section is at least about 0.3% higher than that at thetwo opposing edge sections. Bearing the margin of error in mind, thisgenerally ensures that, at no point in the strip, is the crown such thatthe central section is less than about 0.1% greater in thickness thanthe opposing edge sections. Typically however the manufacturing processis set so that the crown is such that the central section isapproximately 0.5% greater in thickness than the opposing edge sections,but up to 1% or even higher is possible, with 2% the practicablemaximum.

[0005] Creep occurs during coiling, when it may be made easier by theslight warming of the aluminium that often occurs during cold rolling orduring pre-treatment processes such as cleaning or during stoving afterpainting. Creep continues in the coil even at room temperature, untilthe stress is relaxed to the extent that the creep rate becomesinsignificant.

[0006] As each lap of the aluminium strip is coiled under tension aboutthe spool, each new lap imposes an incremental inward pressure on thematerial that has already been coiled onto the spool. This results inthe flatness of the strip varying with respect to its position in thecoil. For example, the strip from the outer laps of the coil (otherwisereferred to as wraps) can buckle along the centre line of the stripwhilst strip from the inner wraps can buckle along its edges. The formerdeviation from flatness is termed ‘long-middle’ whereas the latterdeviation from flatness is termed ‘wavy edges’.

[0007] When the aluminium strip is being coiled onto the spool, thespool is mounted on a mandrel which rotates the spool during the coilingprocedure. Once the coiling of the strip has been completed, the spoolis removed from the mandrel. Unfortunately, especially with fibrespools, the spool can deform under the pressure from the coiled stripwhich can further exacerbate the problems mentioned above with respectto off-flatness. The compressive force from the coil causes the spool toradially displace inwards which makes the inner laps shorter and socauses the tension in the inner laps to be reversed. FIG. 1 is a modelprediction of creep strain (in i units) for a coil on a conventionalspool 24 hours after coiling. The compressive (−ve) strain for the innerlaps at the middle of the strip can be clearly seen along with a largepositive strain either side of the middle region of the strip. Thismodel thus predicts the strip at the inner laps to have wavy edges withquarter pockets. Quarter pockets are formed by the buckling of the stripalong parallel longitudinal lines inboard from each of the longitudinaledges of the strip approximately a distance equal to a quarter of thetotal width of the strip.

[0008] Schnell et al (Metallwissenschaftund Technik vol. 8 August 1986)have described the problem of off flatness and attempted to explainthese effects but have not proposed any solution.

[0009] Attempts have been made to reduce the off-flatness caused bycoiling but these attempts have generally focused on post processing ofthe strip to straighten the strip. However, in JP11-179422 a method isdescribed for controlling the flatness of steel strip material that hasa convex crown which utilises a contoured spool having a concave crown.

[0010] JP 09-057344 and JP 09-076012 both describe similar methods ofwinding steel strip material onto a mandrel. In both cases a narrowsleeve defining a convex crown is fitted on the mandrel and ispositioned centrally of the width of the steel strip being coiled.

[0011] The present invention seeks to provide a system and a method ofcoiling aluminium strip on a spool in such a way as to reduce thedeformation of the strip resulting from creep, and thereby improve theflatness of the strip. The present invention is particularly concernedwith reducing the wavy-edge off-flatness in the inner laps of a coil ofaluminium strip material.

[0012] As already mentioned, a conventional cylindrical spool defines anouter supporting surface for the strip material which is cylindrical inshape. If the strip material were of a constant thickness across itswidth, then the spool would provide a substantially constant supportacross the width of the strip material and the uneven stresses whichcause creep would not arise. However, where the strip has a positivecrown, the conventional spool gives a greater support to the strip atits middle than at its edges, the exact profile of this variationdepending on the shape of the profile across the strip. Theaforementioned JP11-179422 seeks to cater for this by providing that theexternal shape of the spool inversely matches the external shape of thestrip across its width, the purpose being to try to negate the unevenstresses caused by the variation in the thickness of the strip acrossits width, to thus emulate the situation which would occur if the striphad a constant thickness across its width; hence, for a strip having apositive crown the external shape of the spool is concave, andvice-versa.

[0013] In a first aspect of the invention there is provided a system forcoiling of aluminium strip material, said system consisting of a coilassembly comprising a mandrel, a spool removably mounted on said mandreland an aluminium strip material having a positive crown, said coilassembly having a supporting surface on which is to be coiled said stripmaterial, and wherein the coil assembly is adapted so that itssupporting surface provides a support profile in which the supportprovided by that part of the supporting surface which supports the crownis greater than that provided by the remaining part or parts of thesupporting surface during coiling of at least the inner laps of thestrip material.

[0014] The normal natural consequence of the rolling process by whichthe strip material is made is that the crown is positioned approximatelycentrally with respect to the width of the strip material; however,subsequent processing, for example the slitting of a wider strip to formnarrower ones, may result in the crown being off-centre when it iscoiled. The teaching of the present invention can be applied whateverthe position of the crown, but it will be assumed herein that the crownis approximately centrally positioned with respect to the stripmaterial, in which case, the support provided to the central portion ofthe strip material will be greater than that provided to opposing edgeportions of the strip material during coiling of at least the inner lapsof the strip material.

[0015] The support profile of the supporting surface may be provided byadaption of the shape and/or properties of the spool, or by adaption ofthe strip material to be wound thereon, or a combination of both.

[0016] Adaption of the coil assembly to enable its supporting surface toprovide the required support profile may be achieved in a number ofways. For example, the spool may be contoured to define a supportingsurface which has a diameter at a central region which is greater thanat its end regions. Thus, during coiling of the strip material, a largertensile stress is applied to the central region of the strip than to itsend regions, particularly in the inner laps of the coil.

[0017] The interface between the greater diameter in the central regionand the lesser diameter at the end regions may be by way of one or moresteps, or may be a smooth transition, or a combination of both,according to the circumstances. Thus the contour of the supportingsurface may vary from a smooth convex surface, extending across theexpected width of the strip material to be coiled, to a steppedcylindrical surface in which the central region has a greater diameterthan the end regions, the central region having a width less than thewidth of the strip material to be coiled.

[0018] The use of a spool having such a convex supporting surface actsto alter the distribution of stress in the inner laps of the coiledstrip, thereby reducing subsequent creep strain. Using the contouredspool of the present invention, the concentration of coiling tension inthe middle of the strip width arises at the start of coiling. Thisreduces the amount of strip that must be discarded from the inner lapsof a coil where strict flatness requirements apply. In contrast, on anormal plain cylindrical spool, the concentration of coiling tensionarises only after some laps have been coiled. Hence, the presentinvention is of particular benefit when used with aluminium stripmaterials for which there are strict flatness requirements such asmaterials used in lithographic printing.

[0019] The required support profile may be achieved by altering theexternal physical profile of the spool itself, or by adding profilingelements to an otherwise plain cylindrical spool, or a combination ofboth techniques may be used. Thus, for example, a profiling element inthe form of a sleeve may be fitted over the central region of a plaincylindrical spool to increase the effective diameter of the supportingsurface of the spool in its central region. Such a sleeve will have alength which is less than the width of the strip material to be coiled.This arrangement has the advantage that a plain cylindrical spool can beused; such spools can be manufactured very cheaply by simply cutting offsuitable lengths from an elongate tube. Anything more complicated, suchas a profiled tube, is likely to have to be manufactured as anindividual item and is thus much more costly. In the industry, spoolsare regarded as throw-away items and therefore cost is an importantfactor.

[0020] Another way of utilising a plain cylindrical spool is to realisethe aforementioned profiling element as the leading end of the stripmaterial itself, for example by providing that the strip is formed, atits leading end, with a tongue which is narrower in width than theremainder of the strip The tongue has a length, in the longitudinaldirection of the strip, which is approximately equal to thecircumference of the outer surface of the spool. Thus, as coilingcommences, the first lap is formed by the narrow tongue which thuseffectively forms a profiling element as described above. The thicknessof the tongue, and hence the profiling element so formed, isconveniently equal to the thickness of the strip material; if athickness greater than this is required, then the length of the tonguecan be increased to provide two or even more turns, before the fullwidth of the strip commences. Preferably the length of the tongue isequal to n times the outer circumference of the spool, where n is aninteger.

[0021] In an embodiment, the width of the tongue increases from asmaller width to the full width of the strip material during the firstfew laps of the strip material about the coil assembly.

[0022] Another way of adapting the aluminium strip material to providethe required support profile is an arrangement in which a sheet of, forexample, aluminium, is attached, for example by adhesive, mechanicalfixing, welding, or spot welding to a surface of the leading end of thestrip material, said sheet having a width narrower than that of thestrip material, and being centrally located with respect to the width ofthe strip material, said sheet of material being effective, as the stripmaterial is coiled, to provide the spool with an effective outerdiameter at a central region of the spool that is greater than theeffective outer diameter of the spool at opposing end regions of thespool. Preferably said sheet of material has a length, in thelongitudinal direction of the strip material, which is approximatelyequal to n times the outer circumference of the spool, where n is aninteger.

[0023] An alternative way of adapting the spool to provide the requiredsupport profile is to alter the support strength provided by the spoolalong the length of its supporting surface. When the strip is coiledonto the spool, compressive forces act radially inwards on the spool,thus causing compression of the spool material. Conventionally, thespool is constructed with a constant cross section in the direction ofits axis, at least along that part of its length which defines thesupporting surface. This ensures that any distortion of the spool causedby these compressive forces is substantially constant across the widthof the strip material being coiled. If, however, the cross section isnot constant along the axis then the effect of the compressive forceswill be different across the length of the supporting surface. Thistranslates into a different effective support for the strip materialbeing coiled according to its position across the width. Thus, forexample, if the cross section of the centre region of the spool isgreater than at the end regions, then the required support profile canbe achieved even if the supporting surface itself has a conventionalplain cylindrical shape. A similar effect can be achieved by weakeningthe support which the material of the spool is capable of providing incertain select regions by removing material to reduce its strengthwithout necessarily changing the shape of the supporting surface itself.For example, the support which the end regions of the supporting surfaceprovides can be reduced with respect to that provided by the centralregion by cutting slits into the material of the spool to form fingersat the ends, which partially collapse (i.e. move inwards) when the coilis wound onto the spool.

[0024] A further way of adapting the spool so that its supportingsurface exhibits the required support profile is to vary the stiffnessor rigidity of the material of the spool along its length, for exampleby forming the central region of a material having a greater stiffnessor rigidity than the material of the opposing end regions. This can bechanged by altering the inherent stiffness or rigidity of the materialitself, or by locally weakening the material by forming apertures orslits, somewhat in the manner discussed above.

[0025] It has already been mentioned that, in conventional practice, thespool is mounted on a mandrel, the mandrel being caused to rotate thespool during coiling. It is possible to use the mandrel to adapt anotherwise conventional spool to cause its supporting surface to providea support profile which varies along its length in the manner describedabove. Thus, for example, the mandrel may be such as to deform the spoolwhen in place on the mandrel such that the diameter of the supportingsurface of the spool in the central region is greater than that at theopposing end regions. In such a case, the mandrel would normally be ofthe expanding type, whereby it could be collapsed for removal aftercoiling is completed.

[0026] A combination of these various techniques can be used to achievethe desired support profile.

[0027] In an embodiment, the spool is adapted such that the supportprofile of its supporting surface matches, at least approximately, theshape of a graph representing the radial displacement of an outer lap ofa strip material of the same type as that to be coiled, which stripmaterial has been coiled on a conventional right cylindrical spool,after removal of the mandrel.

[0028] In a second aspect the present invention provides a method ofcoiling aluminium strip material having a positive crown wherein thestrip material is fed to a coil assembly comprising a spool and amandrel; the coil assembly is rotated thereby coiling the strip materialabout a supporting surface of the coil assembly; and thereafter themandrel is removed, said method being characterised in that, duringcoiling of at least the inner laps of the coiled strip material, thecoil assembly is adapted so that its supporting surface provides asupport profile in which the support provided by that part of thesupporting surface which supports the crown is greater than thatprovided by the remaining part or parts of the supporting surface.

[0029] In a further alternative, either alone or in combination with theabove aspects of the invention, a tension force is applied to thealuminium strip as it is being coiled. Tension is not applied until theleading end of the strip has become firmly gripped to the spool, thisusually being shortly after the turns begin to overlap at the completionof the first lap. Preferably the initial laps of the strip are coiled ata first higher tension and a second lower tension is applied to laterlaps of the strip as it is being coiled. Thus, most of the coil iscoiled with the strip under a nominal tension, sufficient to hold thecoiled coil in a stable state for storage and transportation. Thissecond (nominal) tension is preferably at least 10% lower than thefirst, higher, tension and is more preferably at least 20% lower. Inaddition, the second tension is preferably no greater than 80% lowerthan the first tension and is more preferably no greater than 50% lower.The coiling tension may be continuously reduced from the higher tensionto the lower tension and this reduction to the lower tension ispreferably performed during the first half of the total laps of thecoil. This is illustrated conceptually in FIG. 17 which shows a shortlevel section (curve a) at a higher tension, followed by the remainderat a lower tension—the nominal tension. The transformation from thehigher tension to the nominal tension may be relatively rapid, as shownby curve a, or may be slower, with or without a shorter section at thehigher tension, as shown by curves b and c. The tension build-upassociated with the first lap is not shown.

[0030] Reference herein to aluminium is to be understood as a referenceto aluminium and its alloys.

[0031] Reference is also made herein to flatness and to off-flatness. Inthe context of this document off-flatness is to be understood to be thedifference in strain across the width of the strip as measured atdifferent positions along the longitudinal or coiling direction of thestrip.

[0032] Embodiments of the present invention will now be described by wayof example with reference to and as shown in the accompanying drawings,in which:

[0033]FIG. 1 illustrates a model prediction of the creep strain for analuminium strip coiled on a conventional spool;

[0034]FIG. 2 is a schematic perspective view of a spool in accordancewith the present invention;

[0035]FIG. 3 illustrates a model prediction of the radial displacementof the outer lap of an aluminium strip coiled on a conventional rightcylindrical spool after removal of the mandrel;

[0036]FIG. 4 illustrates a model prediction of the distribution of hoopstress across the width of three different positions in a coil duringcoiling on a conventional spool, and after removal of the mandrel;

[0037]FIG. 5 illustrates a model prediction of the distribution of hoopstress across the width of the same three laps as for FIG. 4 duringcoiling on a spool, and after removal of the mandrel, in accordance withthe present invention;

[0038]FIG. 6 illustrates a model prediction of the distribution of hoopstress across the width of the same three laps as for FIG. 4 duringcoiling on an alternative spool, and after removal of the mandrel, inaccordance with the present invention;

[0039]FIG. 7A, B, C are diagrammatic plan views of the leading end of analuminium strip to be coiled, showing shaped end sections;

[0040]FIG. 8 is a diagrammatic plan view of the leading end of analuminium strip to be coiled, showing a modified end section.

[0041]FIG. 9 illustrates a model prediction of the creep strain acrossthe width of the first lap 5 mm radially from the spool immediatelyafter coiling for a conventional spool and for a spool in accordancewith the present invention and a spool similar to the prior art spool ofJP11-179422;

[0042]FIG. 10 illustrates a model prediction of the creep strain for analuminium strip coiled on a spool having a centre sleeve in accordancewith the present invention, 24 hours after coiling;

[0043]FIG. 11 illustrates a model prediction of creep strain withrespect to initial coiling tension and spool contour immediately aftercoiling;

[0044]FIG. 12 illustrates a model prediction of creep strain withrespect to initial coiling tension and spool contour 24 hours aftercoiling;

[0045] FIGS. 13 to 16 are graphs of position across strip width againstposition along strip length illustrating the results of various testscarried out on coiled strips; and

[0046]FIG. 17 is a graph to illustrate the variation of applied coilingstress as the coiling proceeds.

[0047] A spool 1 for use in the storage and transportation of aluminiumstrip material is shown in FIG. 2. The spool 1 is approximatelycylindrical but has a central crown region 2 where the outer diameter ofthe spool is greater than at the edge regions 3. The length of the spoolis such as to fully support the strip material, which means in practicethat the spool is at least as long as the width of the strip, and mayindeed be longer; however, under certain circumstances, the spool may bevery slightly shorter—perhaps by up to about 50 mm—than the width of thestrip to meet certain specialist requirements. The outer diameter of thespool increases continuously to a plateau of uniform diameter from theedge regions 3 to the centre region 2. The difference between thediameter of the end and centre regions can be as great as 10 mm or more.For some applications the edge regions 3 can be cut away to leave only anarrow spool supporting just the centre of the coil. Such a narrow spoolor a spool having a very high crown region 2 could mark the inner lapsof the coil. The preferred difference in height between the edge regions2 and the crown 3 is 0.02 to 1.0 mm, preferably 0.05-0.3 mm still morepreferably 0.05 to 0.10 mm.

[0048] The shape of the spool 1 may alternatively match the profileshown in FIG. 3 which is a model prediction of the radial displacementof an outer lap on a right cylindrical spool after removal of themandrel. As can be seen, the maximum displacement of the strip, in thiscase, 0.07 mm, is at the centre of the strip and the displacementrapidly decreases to zero from the maximum over a central regionapproximately 800 mm wide. However, the maximum displacement will dependon the height of the crown on the strip and the number of laps in thecoil. Where the spool 1 has the shape shown in FIG. 3 the distributionof hoop stress while coiling the inner laps of the aluminium strip wouldbe similar to the distribution of hoop stress for the outer laps. Thehoop stress is a measure of the tension force, acting in thecircumferential direction of the coiled strip, per unit cross sectionarea of strip.

[0049] The effect of coiling an aluminium strip on a spool 1 modified inaccordance with the invention is illustrated with reference to FIGS. 4to 6. In FIG. 4 the distribution is shown of hoop stress across thewidth of three laps during coiling on a conventional right cylindricalspool. As can be seen the coiling tension is carried by in excess of themiddle 800 mm of strip width whilst the innermost position is beingcoiled but this is reduced to only 600 mm when coiling the thirdposition. This effect saturates after approximately 50 mm build-up ofcoil. After the mandrel is removed from the spool the reversal of thestress extends over the middle 500 mm of strip width and leaves quarterpockets of residual tension in the strip either side of the largecompressive stress, at the inner position.

[0050] In FIG. 5 a similar distribution of hoop stress is shown for analuminium strip being coiled on a spool having the shape described abovewith reference to FIG. 3. Here it can be seen that the coiling tensionis carried by the middle 500 mm of the strip width throughout coilingand no tension pockets will be formed in the inner position after themandrel is removed from the spool. Thus, using a spool shape that isconvex with a crown across its centre region, a strip with improvedflatness can be achieved. Even a small variation in the outer diameterof the spool at its central region can produce a dramatic effect to thecoil stress.

[0051] Although it may be difficult to construct a spool having theshape described in FIG. 3, shapes capable of achieving similarimprovements in sheet flatness can be easily constructed. For example,an approximately cylindrical deformable spool may be used in conjunctionwith a mandrel that varies in diameter between the centre of the spooland the spool edges. If the mandrel has a positive crown the spooldeforms to a similar crown. Ideally, the mandrel is constructed so thatthe spool is not in contact with the mandrel either side of the centralcrown region.

[0052] However, the preferred spool structure utilises a length of stripto create a raised crown for the centre region of a plain cylindricalspool. For example, a conventional cylindrical spool, having uniformdiameter, is converted by means of a short length of metallic (e.g.aluminium) strip having a gauge of approximately 0.28 mm gauge and awidth of around 525 mm which is wound with one or more turns around thecentre region 2 of the spool to form a sleeve about the centre region ofthe spool. The aluminium strip to be coiled is then wound around theoutside of the converted spool in the usual manner. It will, of course,be appreciated that the sleeve need not be made from a metallic materialand may instead be of natural fibre, plastic or other durable material.Also, as the sleeve is a separate part of the spool it can easily beconstructed to the desired gauge and width. FIG. 6 shows thedistribution of hoop stress for the same three positions using theconverted spool described above and as can be seen the effect of usingthe converted spool is similar to that of FIG. 5. In particular quarterpockets on the inner laps of the strip are avoided. FIG. 6 was producedon the basis of a spool having a rectangular crown 460 mm wide. Therectangular crown concentrates the hoop stress of the inner laps, forexample after 5 mm build-up, into the same width as the stress in thesubsequent laps is concentrated by the coil crown. Thus, the effect ofthe increased spool diameter in the central portion of the strip is toreduce the width-wise range of hoop stress in the inner laps after themandrel has been removed. This can be seen by comparing the hoop stresscurves for the first position in FIG. 4 with the corresponding curve inFIG. 6. The difference comes about because the region of increaseddiameter supports the central part of the coil, leaving the outerregions unsupported and thus with low absolute hoop stress.

[0053] In a further alternative embodiment, illustrated in FIG. 7, aconventional plain cylindrical spool (not shown) may be used for coilingan aluminium strip 10. In order to provide the crown at the centreregion of the spool, the leading end of the strip is shaped to form atongue 11 having a width less than that of the strip 10. With theleading edge 12 of the tongue 11 centred on the spool, the first one ormore laps of the strip build up to form a crown at the centre region ofthe spool. Thereafter, the strip 10 becomes full width and coiling ofthe strip continues in the usual manner. In this way the leading end ofthe strip itself is used to create the convex surface of the spool toensure that the tensile stress is applied to the centre region of theinnermost laps of the strip at its full width. FIG. 7 shows threepossible shapes for tongue 11. In FIG. 7A, the tongue is rectangular inshape, with a substantial step change to full width (although inpractice corners would preferably be rounded to reduce stress). In FIGS.7B and 7C, a gradual transition from the leading edge 12 to full widthis used, thus reducing the likelihood of snatching of the exposedcorners as the strip passes through the processing machinery. Althoughconcave curves are shown in FIGS. 7B and 7C, straight sides could alsobe used, the best shape for the circumstances being determined byexperiment.

[0054] The length l of the tongue should be at least equal to a singleturn around the circumference of the spool; however, if this does notgive sufficient thickness a longer tongue can be used, preferably oflength equal to a multiple of the circumferential length of the spool,since other than a multiple would lead to unbalanced forces duringcoiling.

[0055] In a still further alternative, illustrated in FIG. 8, aconventional plain cylindrical spool (not shown) is used, and the strip10 adapted by attaching to one face, at the leading end, a sheet 13 ofthin material. This material may, for example, be aluminium which isattached by adhesive. It will be seen that, as the strip 10 is coiledaround the spool, the thickness of sheet 13 acts to increase theeffective diameter of the spool in the central section of the width ofthe strip 10, thus giving the same effect as described above. One ormore further sheets (not shown) may be attached on top of sheet 13 toincrease the thickness, as required, and these extra sheets may beattached to the opposite surface of strip 10. The “extra” sheet orsheets thus applied need not necessarily be the same size as sheet 13,but could be smaller to provide a stepped edge or edges to sheet 13.

[0056] The length of sheet 13 in the longitudinal direction of the strip10 will be at least equal to the circumferential length of the spool andpossibly a multiple thereof, as discussed above with reference to thetongue 11 of FIG. 7.

[0057] In FIG. 9 the creep strain across the width of the first position24 hours after coiling is illustrated for a conventional rightcylindrical spool, a spool having a convex (positive) crown, and a spoolhaving edge sleeves. In FIG. 9, creep strain is given in i-units whichare defined as

ε_(r)·10⁵

[0058] where ε_(r) is the relative strain, given by:

ε_(r) =ΔL/L _(a)

[0059] where

[0060] ΔL=change in length

[0061] L_(a)=average of original lengths of all positions across thewidth of the strip

[0062] As can be seen in FIG. 9, for the conventional spool the strainextends over the middle 800 mm of the strip width so that the strip atthe innermost superlap is likely to exhibit wavy edge off-flatness. Fora strip coiled on a convex spool the strain extends over only the middle500 mm of width and will exhibit less wavy edge off flatness. The spoolwith the edge sleeves produces massive differences in strain between thecentre and the edge and consequently a large off flatness. This lattercorresponds approximately to the prior art spool of JP 11 17 94 22.

[0063] In FIG. 10 the flatness change over the entire length of thealuminium strip in terms of creep strain (in i units) is illustrated andmay be compared with FIG. 1 for a conventional spool. Most notably, forthe inner laps the positive strain towards the edges of the strip inFIG. 1 is missing from FIG. 10. Also the magnitude of any wavy edgeeffects is greatly reduced in FIG. 10. FIG. 10 thus illustrates that theoff-flatness effects likely to be found using conventional coilingmethods can be avoided or at least reduced using the contoured spool andthe coiling method described above.

[0064] The positive contours of the spool may also be achieved byweakening the axial ends of the spool. For example, slits may be cutinto the ends of the spool up to a distance of approximately ¼ the widthof the spool which would cause the ends to collapse under thecompressive load of the coil (for example when the ends are notsupported by the mandrel or when the support from the mandrel iswithdrawn) to form a central convex crown. Here too the beneficial shapeis adopted by the spool only after a few laps of the aluminium strip. Ina further alternative, the central region of the spool may beconstructed of a different material to that of the edge regions with thematerial of the central region being more rigid so that as the stripmaterial is coiled onto the spool the edge regions produce a greaterdeflection in response to the compressive load of the laps than thecentral region.

[0065] The above description has focused on utilising a convex spool toreduce the off-flatness effects of a coiled aluminium strip. It is alsopossible to control off-flatness effects through controlling andadjusting the tension of the strip as it is being coiled. To reduceoff-flatness effects the tension applied to the strip must be higher,for example up to 30 MPa, for the initial laps of the coil and then bereduced to a lower tension for the outer laps of the coil. Thisreduction in tension can extend over up to half the entire length of thestrip. However, it is preferable if the reduction in tension is limitedto the first third of the entire strip length.

[0066] The earlier model predictions for a convex spool were allgenerated assuming that the maximum coiling tension for the initial lapswas about twice that of the outer laps the reduction being effected overabout the first 25 mm of build up of the coil (referred to as theconventional practice). In FIGS. 11 and 12 the effect of coiling tensionon the flatness of aluminium strip coiled onto a convex spool isillustrated. In FIG. 11 creep strain along the centre line of the strip,immediately after coiling, is plotted for an aluminium strip coiled ontoa conventional plain spool using conventional practice; onto a convexspool using conventional practice; onto a convex spool using an initialcoiling tension of 10 MPa; and onto a convex spool using an initialcoiling tension of 15 MPa. In the last two cases, the coiling tensionwas decreased exponentially to about half the original value during thefirst 15 mm build up of the coil. As the coil continues to build up, itcan be advantageous to decrease the tension still further to a levelthat does not cause significant creep to occur e.g. to around 10 to 50%of the starting tension. It can be clearly seen from FIG. 11 that theuse of a convex spool in combination with a much higher initial coilingtension greatly increases the creep strain in the strip for the innerlaps of the coil and indeed that the larger the initial tension, thelarger the long middle strain in the inner laps during coiling. In FIG.12, which provides the same examples for comparison but for creep strain24 hours after coiling, it can be seen that the larger the initialcoiling tension the smaller the compressive strain in the inner lapsafter 24 hours. From FIG. 12 for an initial coiling tension of 15 MPa,the strip is flat for the laps very close to the spool and then a wavyedge builds up at around 25 mm.

[0067] Whilst details are given of different structures of spools anddifferent methods of adjusting coil tension for enabling the stress inthe inner laps to be adjusted, these are only examples and the spiritand scope of the present invention is not restricted to the particularexamples given above.

EXAMPLE

[0068] AA1050 sheet cold rolled to a thickness of 0.28 mm and width of1050 mm with a positive crown profile was wound into coils 1750 mm indiameter using the conventional practice. Four coils were made one oneach of the following spools:

[0069] 1) Cylindrical spool (comparative example)

[0070] 2) Cylindrical spool as in (1) but with eight equally spacedslits in each end of the spool extending to the edge of the central 500mm region.

[0071] 3) As in (1) but with a single lap of 0.15 mm thick 500 mm widealuminium strip wound round the centre of the spool

[0072] 4) As in (3) but with a strip 0.3 mm thick

[0073] 24 hours after coiling the coils were unwound and flatnesssamples 4 m long were taken at intervals along the entire length of thesheet. Samples were taken closer together towards the spool end of thecoil than at the start. Flatness was measured by placing the samples ona flat steel table and measuring the levels of any off-flatness,represented as strain in i-units, by means of displacement transducers.The results are plotted in FIGS. 13 to 16 respectively showing contoursof levels of off-flatness for various positions in the coil. The samecontour steps, of 0.25 i-units, have been used for all graphs. From thefigures it will be seen that the crowned spools reduced the level ofoff-flatness by a factor of about 2.5. This is a significantimprovement.

1. A system for coiling of aluminum strip material having a coilassembly comprising a mandrel, a spool removably mounted on the mandreland an aluminum strip material having a positive crown, the coilassembly having a supporting surface for coiling the strip material, thesupporting surface providing a support profile in which support providedby that part of the supporting surface which supports the crown isgreater than that provided by remaining parts of the supporting surfaceduring coiling of inner laps of the strip material.
 2. A system asclaimed in claim 1, wherein the crown is located in a central portion ofthe width of the strip material, and wherein the support provided to thecentral portion of the strip material is greater than that provided toopposing edge portions of the strip material.
 3. A system as claimed inclaim 1, further including at least one tensioning roll and a tensioncontrol device to control tension of the strip material as it is coiledfrom a first higher tension to a second lower tension.
 4. A system asclaimed in claim 1, wherein the spool has length at least equal to thewidth of the strip material.
 5. A system as claimed in claim 4, whereinthe support profile of the supporting surface is provided by adaptationof the spool.
 6. A system as claimed in claim 5, wherein the spool hasan outer diameter at that part of the spool which supports the crownthat is greater than the outer diameter of the spool at one or bothopposing end regions of the spool.
 7. A system as claimed in claim 6,wherein the spool is contoured to have an outwardly projecting crownover said part of the spool.
 8. A system as claimed in claim 7, whereinthe outwardly projecting crown is rectangular.
 9. A system as claimed inclaim 5, wherein the spool is cylindrical and of substantially uniformdiameter, and has slits extending from one or both ends of the spool.10. A system as claimed in claim 9, wherein the slits extendapproximately one quarter of the entire length of the spool.
 11. Asystem as claimed in claim 5, wherein that part of the spool whichsupports the crown is formed of a material having greater rigidity thanthe material on one or both of the opposing end regions of the spool.12. A system as claimed in claim 4, wherein the support profile of thesupporting surface is provided by means separate from the spool.
 13. Asystem as claimed in claim 12, wherein the spool is cylindrical inshape.
 14. A system as claimed in claim 13, wherein the support profileof the supporting surface is provided by an outer sleeve mounted aboutthat part of the spool which supports the crown, the outer sleeve havinga width less than the width of the strip material.
 15. A system asclaimed in claim 14, wherein the outer sleeve is cylindrical and isfitted over the spool so that the spool has an effective outer diameterat that part of the spool that is greater than the effective outerdiameter of the spool at one or both opposing end regions of the spool.16. A system as claimed in claim 13, wherein the support profile of thesupporting surface is provided by shaping the strip material.
 17. Asystem as claimed in claim 16, wherein the leading end of the stripmaterial is formed as a tongue having a width narrower than the width ofthe strip material, the tongue being effective, as the strip material iscoiled, to provide the spool with an effective outer diameter at thatpart of the spool which supports the crown that is greater than theeffective outer diameter of the spool at one or both opposing endregions of the spool.
 18. A system as claimed in claim 17, wherein thelength of the tongue, in the longitudinal direction of the stripmaterial, is approximately equal to n times the outer circumference ofthe spool, wherein is an integer greater than zero.
 19. A system asclaimed in claim 16, wherein a sheet of material is attached to asurface of the leading end of the strip material, the sheet having awidth narrower than that of the strip material, the sheet of materialbeing effective, as the strip material is coiled, to provide the spoolwith an effective outer diameter at that part of the spool whichsupports the crown that is greater than the effective outer diameter ofthe spool at one or both opposing end regions of the spool.
 20. A systemas claimed in claim 19, wherein the sheet of material has a length, inthe longitudinal direction of the strip material, which is approximatelyequal to n times the outer circumference of the spool, where n is aninteger greater than zero.
 21. A system as claimed in claim 20, whereinthe sheet is made of aluminum.
 22. A system as claimed in claim 13,wherein the support profile of the supporting surface is provided by alength of material which is wound one or more times around the spoolprior to coiling, the length of material having a width narrower thanthat of the strip material, the length being effective to provide thespool with an effective outer diameter at that part of the spool whichsupports the crown that is greater than the effective outer diameter ofthe spool at one or both opposing end regions of the spool.
 23. A systemas claimed in claim 22, wherein the support profile of the supportingsurface matches, at least approximately, the shape of a graphrepresenting the variation of the radial displacement of an outer lap ofa strip material of the same type as that to be coiled, which stripmaterial has been coiled on a conventional plain cylindrical spool. 24.A method of coiling aluminum strip material having a positive crown,comprising feeding strip material to a coil assembly comprising a spooland a mandrel, rotating the coil assembly to coil the strip materialabout a supporting surface of the coil assembly, and thereafter removingthe mandrel, wherein, during coiling of at least the inner laps of thecoiled strip material, the supporting surface provides a support profilein which support for the crown exceeds that provided by the remainingpart of the supporting surface.
 25. A method as claimed in claim 24,wherein the crown is located in a central portion of the width of thestrip material, and wherein the support provided to the central portionof the strip material exceeds that provided to opposing edge portions ofthe strip material.
 26. A method as claimed in claim 25, wherein, whilethe initial laps of the strip material are being coiled, a first highertension is applied to the strip material and a second lower tension isapplied to later laps of the strip material during coiling.
 27. A methodas claimed in claim 25, wherein the mandrel deforms the spool when inplace within the spool such that the outer diameter of the spool in thatpart which supports the crown is greater than the outer diameter of thespool at one or both opposing end regions, and wherein the mandrel iscollapsible for removal of the coil.
 28. A method as claimed in claim26, wherein, prior to coiling, a length of material is wound one or moretimes around the spool, the length of material having a width narrowerthan that of the strip material to provide an effective diameter of thespool which is greater at that part thereof which supports the crownthan at one or both of its opposing end regions.
 29. A method as claimedin claim 28, wherein the leading end of the strip material is formedwith a tongue having a width which is less than that of the stripmaterial, and wherein coiling commences with the tongue so that thetongue effectively profiles the supporting surface of the spool, so asto define an effective diameter at that part thereof which supports thecrown greater than that at one or both of its opposing end regions. 30.A method as claimed in claim 29, wherein the width of the tongueincreases from a smaller width to the full width of the strip materialduring the winding of the first few laps of the strip material about thecoil assembly.
 31. A method as claimed in claim 26, wherein a sheet ofmaterial is attached to a surface of the leading end of the stripmaterial, the sheet having a width narrower than that of the stripmaterial, the sheet of material being effective, as the strip materialis coiled, to provide the spool with an effective outer diameter at thatpart of the spool which supports the crown that is greater than theeffective outer diameter of the spool at one or both opposing endregions of the spool.